CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Process Technology for Cast Iron Mold Cutting Edges
Literature Overview
The paper by Bai Ruixue from Guangzhou Honda Automobile Co., Ltd., published in Die & Mould Industry in 2005, addresses the overlay welding of hardfacing materials onto the cutting edges of cast iron molds used in automotive stamping operations. Cast iron molds offer excellent cost-effectiveness for high-volume stamping operations but suffer from rapid wear at the cutting edges due to the combination of high contact pressure, abrasive contact with steel sheet, and thermal cycling. The overlay welding approach provides a reliable method for restoring worn cutting edges and extending mold service life.
Background and Technical Context
In automotive sheet metal stamping, die sets are subjected to millions of stamping cycles, with the cutting edges experiencing:
- Peak contact stresses of 2000–3000 MPa during shearing operations
- Sliding friction against carbon steel sheet (typically 0.5–2.0 mm thick)
- Adhesive wear from material transfer between die and sheet
- Thermal loading from frictional heat generation
- Impact loading from high-speed stamping operations
The base mold material, typically grey cast iron (HT200, HT250) or ductile iron (QT500-7, QT600-3), provides adequate bulk properties but insufficient surface hardness and wear resistance for extended service. The cutting edge hardness requirement is typically 60–70 HRC, far exceeding the as-cast hardness of 180–250 HB.
Core Technical Content
Hardfacing Alloy Selection for Cutting Edge Applications
The selection of overlay welding alloy for cast iron mold cutting edges requires careful consideration of the specific wear mechanisms:
| Hardfacing Type | Composition Range | Hardness (HRC) | Wear Mechanism Addressed |
|---|---|---|---|
| Cr-Cr2C | 5-12% Cr, 0.5-1.5% C | 58-65 | Abrasive, adhesive |
| Cr-V | 8-12% Cr, 2-4% V | 60-68 | Abrasive, high impact |
| Ni-Cr-C | 6-8% Cr, 1.2-1.8% C | 62-68 | Abrasive, moderate impact |
| WC-Co | 25-60% WC, balance Co | 65-75 | Severe abrasive |
| High-speed steel type | W, Mo, V, Cr, C | 60-65 | Combined wear |
Welding Process Selection and Parameters
Cast iron presents unique challenges for welding due to:
- High carbon and silicon content promoting graphite formation
- Poor ductility leading to cracking susceptibility
- Thermal expansion mismatch between weld metal and base
The following processes are suitable for cast iron mold cutting edge overlay:
- Flux-cored Arc Welding (FCAW) – Good for thicker deposits, moderate penetration, lower dilution
- Submerged Arc Welding (SAW) – High deposition rate, good for large flat areas
- Gas Metal Arc Welding (GMAW) – Good control, suitable for complex geometries
- Oxy-fuel welding – Low heat input, suitable for thin sections and repair
| Parameter | Recommended Value | Purpose |
|---|---|---|
| Preheating temperature | 300–400 °C | Reduce thermal gradient, prevent cracking |
| Interpass temperature | 250–350 °C | Maintain ductility, prevent cracking |
| Post-weld cooling | Slow (insulated or furnace) | Prevent white cast iron formation |
| Welding current | 200–350 A (GMAW) | Adequate penetration without excessive dilution |
| Layer thickness | 2–4 mm per pass | Control heat input per pass |
| Total overlay thickness | 6–12 mm | Adequate wear life |
Defect Analysis and Countermeasures
The most common defects encountered in overlay welding of cast iron molds include:
1. Cracking
| Crack Type | Location | Cause | Countermeasure |
|---|---|---|---|
| Hot cracking | Weld metal | High S, P content; solidification shrinkage | Low-S filler metal; controlled cooling |
| Cold cracking | HAZ | Hydrogen embrittlement; high restraint | Preheat; low-hydrogen process |
| Graphite cracking | Base metal near weld | Graphite plate fracture during thermal cycling | Adequate preheat; slow cooling |
2. Excessive Dilution
High dilution of the cast iron base into the hardfacing layer reduces the hardness and wear resistance of the overlay. Countermeasures include:
- Using a multi-layer approach with a transition layer
- Selecting filler metals with higher alloy content to compensate for dilution
- Controlling heat input to minimize penetration into the base
3. Poor Bonding
Inadequate fusion at the overlay-base interface results in delamination during service. This is prevented by:
- Ensuring clean, oxide-free base surface (grinding or machining)
- Using adequate preheating to ensure proper fusion
- Applying sufficient first-layer thickness to establish a robust bond
Engineering Practice at Guangzhou Honda
The industrial context of this research – Guangzhou Honda automobile manufacturing – provides valuable insight into the practical requirements for mold maintenance:
- Production continuity – mold downtime directly impacts production output, making rapid repair and restoration critical
- Cost management – overlay welding repair costs are typically 10–20% of new mold fabrication costs
- Quality consistency – restored cutting edges must maintain the same dimensional accuracy and surface finish as new molds
- Cycle time – the overlay welding and post-processing (grinding) must be completed within acceptable maintenance windows
A typical repair cycle involves:
- Removal of the worn cutting edge material by grinding or machining
- Surface preparation and cleaning
- Application of overlay weld layers (2–4 passes)
- Heat treatment if required
- Precision grinding to final cutting edge geometry
- Surface finishing and inspection
Key Reflections
This work highlights several important aspects of industrial overlay welding practice:
- The economic case for overlay welding repair is compelling – extending mold life by 3–5 times through periodic re-overlay is far more cost-effective than periodic mold replacement
- The challenges of welding to cast iron base materials require careful process control, particularly regarding thermal management
- The integration of overlay welding into the mold maintenance workflow requires coordination between welding, machining, and quality inspection operations
The automotive industry context also emphasizes the importance of dimensional accuracy after overlay welding, as cutting edge geometry directly affects stamping quality and part dimensional tolerance.
Summary
The overlay welding of cast iron mold cutting edges represents a well-established industrial practice that significantly extends mold service life in high-volume automotive stamping operations. The key to successful implementation lies in appropriate hardfacing alloy selection based on the specific wear mechanisms, careful thermal management to prevent cracking in the cast iron base, and precision post-processing to restore cutting edge geometry. For engineers involved in die and mold manufacturing, this work provides practical guidance on procedure development, defect prevention, and integration of overlay welding into maintenance workflows for cast iron molds in demanding automotive applications.